Solid adsorbents are commonly used to remove contaminants from hydrocarbon streams such as olefins, natural gas and light hydrocarbon fractions. Since these streams can contain different contaminants, more than one adsorbent or adsorbent bed are needed to sufficiently purify the stream so that it can be used in the desired process. Contaminants which can be present in these streams include H2O, CO, O2, CO2, COS, H2S, NH3, AsH3, PH3, Hg, methanol, mercaptans and other S- or O-containing organic compounds.
While various adsorbents can remove one or more contaminants, they can also remove and/or promote reactions of the desired hydrocarbon. For example, faujasite type zeolites, e.g. zeolite 13X, are good adsorbents for sulfur and oxygenate compounds. However, they are also good adsorbents for olefins resulting in heat generation that can lead to high temperature rise and/or run-away reactions. Additionally, the zeolite's residual surface reactivity can cause oligomerization and polymerization during regeneration. This leads to fouling and performance deterioration.
In attempts to remedy this problem, there are reports in the art where zeolites have been mixed with alumina. U.S. Pat. No. 4,762,537 discloses the use of an adsorbent comprising zeolite Y and alumina to remove HCl from a hydrogen stream. In U.S. Pat. Nos. 4,686,198 and 4,717,483 it is disclosed that a mixture of alumina and sodium Y zeolite can remove ammonia, sulfides and organic impurities from waste water. The sodium Y zeolite contains at least 12.7 wt. % Na2O. The same adsorbent is also used to reduce the acidity and moisture content of used organophosphate functional fluids, see U.S. Pat. No. 4,751,211. The use of alumina with alkali or alkaline earth metal for removing HCl and other contaminants is disclosed in U.S. Pat. No. 6,013,600.
In U.S. Pat. No. 6,632,766, to UOP, there is described an improved adsorbent which can remove multiple contaminants from various hydrocarbon streams. This adsorbent comprises a zeolite, alumina and a metal component which is present in an amount of at least 10 mole % of the stoichiometric amount of metal (expressed as the oxide) needed to compensate for the negative charge of the zeolite lattice. In this patent, the metal component, in particular, sodium, is added to both the zeolite and alumina components.
In the discovery of the patent immediately above, UOP found that adding Na (broadly Group 1A/2A metals) to a zeolite/alumina composited adsorbent greatly reduces the reactivity of olefins/oxygenates over the adsorbent as evidenced by a reduction of oligermization. However, not shown in the patent is a loss in methanol capacity (a representative oxygenate) on the adsorbent. The present inventors have determined that the highest reduction in reactivity can be attained for a given loss in oxygenate capacity by placing the sodium initially only on the zeolite portion of the adsorbent.